This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-212916, filed Jul. 13, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a heat resistant steel casting useful as a material of a steam turbine casing and as a material of a steam turbine valve body and to a method of manufacturing the same.
A low alloy heat resistant steel casting such as a 1.25Cr-0.5Mo cast steel or a 1Cr-1Mo-0.25V cast steel is widely used as a heat resistant steel casting material used for forming a steam turbine casing or a steam turbine valve body in a thermal power station.
On the other hand, in the thermal power station in recent years, the temperature elevation of the steam proceeds rapidly. In accordance with the temperature elevation of the steam, the change of the material of the high temperature member to a high Cr heat resistant steel casting is being vigorously promoted. The high Cr heat resistant steel casting of this kind is disclosed in, for example, Japanese Patent Publication (KOKOKU) No. 4-53928 and Japanese Patent Publication (KOKOKU) No. 3-80865. Since the high Cr heat resistant steel casting exhibits a high mechanical strength and an excellent resistance to the high temperature environment, it is possible to suppress the increase in the thickness of the high temperature member in spite of the elevation of the steam temperature. Also, since it is possible to suppress the thermal stress in the start-up and stop of the steam turbine, the steam turbine can be operated efficiently.
In recent years, the thermal power station is required to exhibit an excellent economical advantage in addition to a high thermal efficiency. Therefore, it is absolutely necessary for the material of the thermal power station to exhibit mechanical properties and manufacturing properties equal to or higher than those of the conventional material and to be excellent in economy. The material meeting these requirements includes, for example, the steel disclosed in Japanese Patent Disclosure (KOKAI) No. 2-217438 and Japanese Patent Disclosure (KOKAI) No. 8-269616.
However, the material of a high temperature member manufactured as a thick cast article is required to exhibit high temperature strength characteristics and economic properties superior to those of the steels disclosed in JP ""438 and JP ""616 quoted above.
An object of the present invention, which has been achieved in view of the situation described above, is provide a heat resistant steel casting exhibiting mechanical properties excellent under an environment in which a high temperature steam flows and excellent in economical properties and a method of manufacturing the particular heat resistance cast steel.
As a result of an extensive research on a low alloy heat resistant steel casting fully comparable to the high Cr steel casting in the high temperature strength characteristics and advantageous in economy, the present inventors have arrived at the present invention summarized below.
According to an aspect of the present invention, there is provided a heat resistant steel casting, comprising C in an amount of 0.15 to 0.3 mass %, Si in an amount of 0.1 to 0.30 mass %, Mn in an amount of 0.01 to 0.1 mass %, Cr in an amount of 2.0 to 2.5 mass %, Mo in an amount of 0.3 to 0.8 mass %, V in an amount of 0.23 to 0.3 mass %, W in an amount of 1.6 to 2.6 mass %, N in an amount of 0.005 to 0.03 mass %, B in an amount of 0.001 to 0.004 mass %, impurity elements including Ni not larger than 0.2 mass %, P not larger than 0.03 mass % and S not larger than 0.01 mass %, B equivalent determined by formula (1) given below being not larger than 0.02 mass %, Mo equivalent determined by formula (2) given below falling within a range of between 1.4 mass % and 2.0 mass %, and C equivalent determined by formula (3) given below being not smaller than 0.65 mass %, and balance of iron and unavoidable impurities:
B equivalent=B+0.5Nxe2x80x83xe2x80x83(1)
Mo equivalent=Mo+0.5Wxe2x80x83xe2x80x83(2)
C equivalent=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/14+V/14xe2x80x83xe2x80x83(3)
wherein a precipitated phase consisting of a M23C6 type carbide, a M7C3 type carbide, and MX type carbonitride is a texture finely precipitated in a matrix phase, and a ratio of the precipitated phase to the matrix phase falls within a range of between 0.6 and 1.0 mass %.
In this case, it is possible for the Nb equivalent determined by formula (4) given below to be not larger than 0.15%, with the V content set at 0.23 to 0.27 mass %, and with Nb content set at 0.01 to 0.06 mass %:
Nb equivalent=Nb+0.4Cxe2x80x83xe2x80x83(4)
It is also possible to set the V content at 0.23 to 0.27 mass % and to set the Ti content at 0.005 to 0.01 mass %.
Further, it is possible to set the V content at 0.25 to 0.3%.
According to another aspect of the present invention, there is provided a heat resistant steel casting, comprising C in an amount of 0.15 to 0.3 mass %, Si in an amount of 0.1 to 0.30 mass %, Mn in an amount of 0.4 to 0.7 mass %, Cr in an amount of 2.0 to 2.5 mass %, Mo in an amount of 0.3 to 0.8 mass %, V in an amount of 0.23 to 0.3 mass %, W in an amount of 1.6 to 2.6 mass %, N in an amount of 0.005 to 0.03 mass %, B in an amount of 0.001 to 0.004 mass %, impurity elements including Ni in an amount not larger than 0.5 mass %, P in an amount not larger than 0.03 mass % and S in an amount not larger than 0.01 mass %, B equivalent determined by formula (1) given below being not larger than 0.02 mass %, Mo equivalent determined by formula (2) given below falling within a range of between 1.4 mass % and 2.0 mass %, and C equivalent determined by formula (3) given below being not smaller than 0.65 mass %, and balance of iron and unavoidable impurities,
wherein a precipitated phase consisting of a M23C6 type carbide, a M7C3 type carbide, and MX type carbonitride is a texture finely precipitated in a matrix phase, and a ratio of the precipitated phase to the matrix phase falls within a range of between 0.6 and 1.0 mass %:
B equivalent=B+0.5Nxe2x80x83xe2x80x83(1)
Mo equivalent=Mo+0.5Wxe2x80x83xe2x80x83(2)
xe2x80x83C equivalent=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/15+V/14xe2x80x83xe2x80x83(3)
In this case, it is possible for the Nb equivalent determined by formula (4) given below to be not larger than 0.15%, with the V content set at 0.23 to 0.27 mass %, and with Nb content set at 0.01 to 0.06 mass %:
Nb equivalent=Nb+0.4Cxe2x80x83xe2x80x83(4)
It is also possible to set the V content at 0.23 to 0.27 mass % and to set the Ti content at 0.01 to 0.025 mass %.
Further, it is possible to set the V content at 0.25 to 0.3%.
The function of each of the components described above and the reasons for specifying the composition are as described in items (a) to (p) described below. In the following description, xe2x80x9c%xe2x80x9d represents the mass % unless otherwise specified.
(a) C: 0.15 to 0.3%
Carbon (C) serves to ensure the hardenability, to suppress the ferrite formation, and to precipitate as a carbide or carbonitride contributing to reinforcement of precipitation. In ensuring the mechanical properties of a thick portion in casting, particularly, a large lump, it is important to ensure the hardenability and to suppress the ferrite formation. If the C content is less than 0.15%, these functions are unlikely to be performed sufficiently. On the other hand, if the C content exceeds 0.3%, the agglomeration of the precipitated carbide tends to be promoted and the welding properties tend to be lowered.
(b) Si: 0.1 to 0.3%
Silicon (Si) serves to perform the function of a deacidifying agent, to ensure a good casting properties, and to enhance the resistance to the steam oxidizing characteristics. If the Si content is lower than 0.1%, these functions tend to fail to be performed sufficiently. On the other hand, if the Si content exceeds 0.3%, the toughness is lowered so as to promote the brittleness.
(c) Mn: 0.01 to 0.1% or 0.4 to 0.7%
Manganese (Mn) serves to perform the function of a desulfurizing agent. If the Mn content is lower than 0.01%, it is difficult to obtain a sufficient desulfurizing effect. On the other hand, if the Mn content exceeds 0.1%, the creep strength tends to be lowered.
Where the heat resistant steel casting of the present invention is used as a large and thick part, it is desirable to increase the Mn addition amount because the ferrite forming tendency is increased in the thick portion by the reduction in the cooling rate in the hardening step. In order to suppress completely the ferrite formation in the thick portion, it is necessary to add Mn in an amount not smaller than 0.4%. In this case, the creep strength of the cast steel having Mn added thereto in an amount of at least 0.4% is slightly lower than that of the cast steel having Mn added thereto in an amount of 0.01 to 0.1%. However, it is possible to avoid a marked reduction in the creep strength, if the Mn addition amount is not larger than 0.7%.
(d) Cr: 2.0 to 2.5%
Chromium (Cr) serves to improve the oxidation resistance and the corrosion resistance and, at the same time, precipitates as a precipitated material contributing to the reinforcement of the precipitation. If the Cr content is lower than 2.0%, these functions tend to fail to be performed sufficiently on the other hand, if the Cr content exceeds 2.5%, the toughness and the texture stability tend to be lowered.
(e) Mo: 0.3 to 0.8%
Molybdenum (Mo) serves to contribute to the reinforcement of a solid solution and precipitates as a carbide so as to contribute to the reinforcement of the precipitation. If the Mo content is lower than 0.3%, these functions tend to fail to be performed sufficiently. On the other hand, if the Mo content exceeds 0.8%, the toughness tends to be lowered, and the ferrite formation tends to be promoted.
(f) W: 1.6 to 2.6%
Like Mo, tungsten (W) serves to contribute to the reinforcement of a solid solution and precipitates as a carbide so as to contribute to the reinforcement of the precipitation. If the cast steel contains W together with Mo, the function of reinforcing the solid solution is rendered more prominent.
In order to maintain a high W content forming a solid solution over a long period of time, it is necessary for the W content to be at least 1.6%. However, if the w content exceeds 2.6%, the toughness tends to be lowered and the ferrite formation tends to be promoted.
(g) B: 0.001 to 0.004%
Boron (B) serves to enhance the hardenability and serves to stabilize the carbonitride precipitated in the crystal boundary and in the vicinity thereof even under high temperatures so as to suppress the enlargement and coarsening of the precipitated carbonitride. If the B content is lower than 0.001%, these functions tend to fail to be performed sufficiently. On the other hand, if the B content exceeds 0.004%, the weldability tends to be impaired.
(h) N: 0.005 to 0.03%
Nitrogen (N) forms a solid solution within the matrix phase so as to contribute to the reinforcement of the solid solution and also forms a nitride or carbonitride so as to contribute to the reinforcement of the precipitation. If the N content is 0.005%, these functions tend to fail to be performed sufficiently. On the other hand, if the N content exceeds 0.03%, the enlargement and coarsening of the nitride or carbonitride are promoted so as to lower the creep strength and to promote formation of large and coarse products. It is more desirable for the N content to fall within a range of between 0.01% and 0.025%. Where the N content falls within the preferred range noted above, the texture can be further stabilized so as to further improve the creep strength.
(i) V: 0.23 to 0.3%
(i-1) V: 0.23 to 0.27% (Where V is Added Together with Nb or Ti)
(i-2) V: 0.25 to 0.3% (Where V is Added Singly)
(i-3) V: 0.23 to 0.25% (where V is Added Together With Ti and the Ti Addition Amount is 0.01 to 0.025%)
Vanadium (V) is precipitated as a fine carbonitride so as to contribute to the reinforcement of the precipitation. Where V is added together with niobium (Nb) or titanium (Ti) referred to herein later, a carbonitride of Nb or Ti is also formed in addition to the carbonitride of V so as to supplement the function of reinforcing the precipitation performed by the carbonitride of V. In the present invention, V is added in an amount of at least 0.23% in the case where Nb or Ti is added together with V. In this case, it is possible to permit precipitation of carbonitride of V at a high density and in an appropriate amount together with precipitation of carbonitride of Nb or Ti. As a result, it is possible to suppress the restoration. However, if the V content exceeds 0.27% in the case of adding Nb or Ti together with V, the carbonitride of v tends to be unduly enlarged and coarsened.
Where V is added together with Ti, it is possible to ensure a sufficient amount of precipitation by suppressing the V content to 0.25% or less and by increasing the Ti content.
On the other hand, where V is added singly without adding Nb or Ti, it is necessary to increase the V addition amount, compared with the case where Ti or Nb is added together with V, in order to permit carbonitride of V to be precipitated in an additional amount corresponding to the precipitated amount of carbonitride of Nb or Ti. Therefore, the V content is defined to be 0.25% to 0.3% in the case of adding V singly.
(j) Nb: 0.01 to 0.06%
Like V described above and like Ti that is to be described herein later, Nb permits precipitation of fine carbonitride so as to contribute to the reinforcement of precipitation. If the Nb content is lower than 0.01%, the function described above tends to fail to be performed sufficiently. If the Nb content exceeds 0.06%, however, large and coarse carbonitride is precipitated in a large amount so as to fail to perform the function of reinforcing the precipitation.
(k) Ti: 0.005 to 0.01% or 0.01 to 0.025%
Titanium (Ti) performs a deacidifying function and is precipitated as a fine carbonitride so as to contribute to the reinforcement of precipitation. These functions can be performed sufficiently where the Ti content is not lower than 0.005%. However, if the Ti content exceeds 0.01% in the case where V is added together with Ti, large and coarse carbonitride tends to be precipitated in a large amount so as to fail to perform the function of reinforcing the precipitation.
However, where the amount of V, which is added together with Ti, is suppressed to 0.25% or less, it is effective to add Ti in an amount exceeding 0.01% because the precipitation reinforcing function of the fine carbonitride is effectively exhibited.
(l) Other elements
It is desirable for the content of the unavoidable impurities other than the components described above and the main component of Fe to be as low as possible. Particularly, it is unavoidable for the impurity elements such as P, S and Ni to enter the cast steel from the raw materials. It is certainly possible to decrease the contents of these unavoidable impurities by the strict selection of the raw materials and by employment of a highly improved dissolving and steel manufacturing technologies. However, these measures are not recommendable in view of economy. Under the circumstances, the Ni content is set at 0.2% or less, the P content is set at 0.03% or less, and the S content is set at 0.01% or less.
Where the heat resistant steel casting of the present invention is used as a large and thick part, it is desirable to increase the Ni addition amount because the ferrite forming tendency is increased in the thick portion by the reduction in the cooling rate in the hardening step. Also, where it is intended to obtain an economical advantage, it is effective to set the limited amount of Ni mixed in the raw material at a high value, though the creep strength tends to be lowered if the Ni amount exceeds 0.5%. Under the circumstances, it is desirable for the Ni amount to be not larger than 0.2% or not larger than 0.5%, for the P amount to be not larger than 0.03%, and for the S amount to be not larger than 0.01%.
(m) B +0.5N xe2x89xa60.02%
B tends to perform reaction with, particularly, N to form boron nitride. The resultant boron nitride remains in the cast lump in the form of a band or a lump so as to deteriorate the mechanical properties. In the present invention, the sum of the boron content and 0.5 time the N content is defined as the B equivalent. The upper limit of the B equivalent is set at 0.02% in the present invention so as to suppress formation of the BN compound.
(n) 1.4%xe2x89xa6Mo+0.5Wxe2x89xa62.0%
As already described, the function of reinforcing the solid solution is rendered prominent by allowing the cast steel to contain both Mo and W. In the present invention, the sum of the Mo content and 0.5 time the W content is defined as the Mo equivalent, and the Mo equivalent is defined to fall within a range of between 1.4% and 2.0%. Where the Mo equivalent falls within the range noted above, the function of reinforcing the solid solution is rendered prominent, and the ferrite formation can be effectively suppressed.
(o) 0.65xe2x89xa6C+Mn/6+Si/24+Ni/40+Cr/5+Mo/15+V/14
As already described in conjunction with the C content, in improving the mechanical properties of the thick cast product, it is important to ensure the hardenability and to suppress the ferrite formation. In the present invention, the C content is defined to fall within the range described in item (a). Also, the value obtained from formula (3) of C+Mn/6+Si/24+Ni/40+Cr/5+Mo/15+V/14 is defined as the C equivalent, and the lower limit of the C equivalent is set at 0.65%. As a result, the hardenability can be ensured without impairing the weldability. Also, the ferrite formation can be suppressed.
(p) Nb+0.4Cxe2x89xa60.15%
It is known to the art that, in the case of adding Nb, large and coarse Nb carbide is precipitated when a large cast lump is coagulated, and that, where the Nb carbide remains in the cast lump, the mechanical properties are adversely affected. In the present invention, the sum of the Nb content and 0.4 time the C content is defined as the Nb equivalent. In the case of adding Nb, the Nb equivalent is defined to be 0.15% or less so as to suppress formation of large and coarse Nb carbide.
The heat resistant steel casting of the present invention is a texture in which a M23C6 type carbide, a M7C3 type carbide and an MX type carbonitride are finely precipitated in the matrix phase. In this case, M represents one kind of an element or a combination of at least two kinds of elements selected from the group including Cr, Mo, W, V and Nb, and X represents an element such as C or N. In the present invention, the mass ratio of the precipitated phase consisting of the M23C6 type carbide, M7C3 type carbide and MX type carbonitride is defined to fall within a range of between 0.6 to 1.0%. The reasons for the definition are as follows.
Each of the precipitated materials is precipitated in the tempering step included in the manufacturing method. If the ratio of the precipitated phase to the matrix phase is set at 0.6 mass % or less, it is difficult to satisfy both the creep strength and the Charpy impact strength. On the other hand, if the ratio noted above exceeds 1.0 mass %, the elements constituting the MX type carbonitride, which is newly precipitated from the matrix phase during use under high temperatures so as to contribute to the retention of the creep strength, are depleted, making it difficult to stabilize the creep strength characteristics under high temperatures.
The heat resistant steel casting of the present invention described above, which is a low alloy cast steel, exhibits excellent characteristics when used as a material of the steam turbine casing and a steam turbine valve body which are exposed to the maximum temperature of 538xc2x0 C. during the normal operation, and has a creep rupture strength higher than that of the conventional 1% CrMov low alloy heat resistant steel casting. Therefore, if the heat resistant steel casting of the present invention is used for manufacturing a steam turbine casing and a steam turbine valve body, which are exposed to the maximum temperature of 538xc2x0 C. during the normal operation, it is possible to decrease the thickness of the wall of the vehicle chamber and the valve box. To be more specific, the wall thickness can be decreased to about 75% of the wall thickness in the case of using the conventional 1% CrMoV low alloy heat resistant steel casting.
It should also be noted that the heat resistant steel casting of the present invention can be used in place of the conventional high Cr heat resistant steel casting as a material of the steam turbine casing and the steam turbine valve body which are exposed to the maximum temperature of 566xc2x0 C. during the normal operation. It is also possible to use the heat resistant steel casting of the present invention as a material of the steam turbine casing exposed to the maximum temperature of 593xc2x0 C. during the normal operation. As a result, the raw material cost can be markedly saved because the heat resistant steel casting of the present invention is a low alloy, though it is necessary to increase the wall thickness by about 25%, compared with the use of the conventional high Cr heat resistant steel casting. It follows that the vehicle chamber and the valve box noted above can be manufactured with a manufacturing cost lower than that in the past.
Further, it is possible to use in combination the heat resistant steel casting of the present invention and the conventional high Cr heat resistant steel casting in manufacturing the steam turbine casing that is exposed to the maximum temperature of 593xc2x0 C. during the normal operation. To be more specific, the heat resistant steel casting of the present invention is used for forming the high temperature steam inlet portion of the steam turbine casing, which is exposed to steam having a temperature of 570xc2x0 C. or more, and the conventional high Cr heat resistant steel casting or a low alloy heat resistant steel casting for forming the other portions. In this case, the member made of the heat resistant steel casting of the present invention is allowed to abut against and welded to the member made of the conventional high Cr heat resistant steel casting or the low alloy heat resistant steel casting so as to manufacture the desired steam turbine casing.
The high Cr heat resistant steel casting used in combination with the heat resistant steel casting of the present invention comprises, for example, C in an amount of 0.12 to 0.16%, Si in an amount of 0.2 to 0.35%, Mn in an amount of 0.5 to 0.7%, Ni in an amount of 0.3 to 0.6%, Cr in an amount of 9.6 to 10.6%, Mo in an amount of 0.7 to 1.0%, V in an amount of 0.2 to 0.35%, Nb in an amount of 0.07 to 0.13%, N in an amount of 0.03 to 0.06%, P in an amount of 0.02% or less, S in an amount of 0.02% or less, Al in an amount of 0.01% or less, and the balance of iron and unavoidable impurities.
On the other hand, the low alloy heat resistant steel casting used in combination with the heat resistant steel casting of the present invention comprises, for example, C in an amount of 0.12 to 0.18%, Si in an amount of 0.2 to 0.6%, Mn in an amount of 0.5 to 0.9%, Cr in an amount of 1.0 to 1.5%, Mo in an amount of 0.9 to 1.2%, V in an amount of 0.2 to 0.35%, P in an amount of 0.02% or less, S in an amount of 0.012% or less, Ni in an amount of 0.5% or less, Al in an amount of 0.01% or less, and the balance of iron and unavoidable impurities.
How to manufacture the heat resistant steel casting of the present invention will now be described.
The method of manufacturing the heat resistant steel casting of the present invention comprises the steps of retaining a cast material comprising C in an amount of 0.15 to 0.30 mass %, Si in an amount of 0.1 to 0.3 mass %, Mn in an amount of 0.01 to 0.1 mass %, Cr in an amount of 2.0 to 2.5 mass %, Mo in an amount of 0.3 to 0.8 mass %, V in an amount of 0.23 to 0.3 mass %, w in an amount of 1.6 to 2.6 mass %, N in an amount of 0.005 to 0.03 mass %, B in an amount of 0.001 to 0.004 mass %, the B equivalent defined by formula (1) below being 0.02 mass % or less, the Mo equivalent defined by formula (2) given below being 1.4 to 2.0 mass %, and the C equivalent defined by formula (3) given below being 0.65 mass % or more, impurity elements including Ni in an amount of 0.2 mass % or less, P in an amount of 0.03 mass % of less, and S in an amount of 0.01 mass %, and the balance of iron and unavoidable impurities, to fall within a temperature range of between 1030xc2x0 C. and 1070xc2x0 C., followed by quenching to the heated cast material, and tempering the cast material at 680 to 740xc2x0 C.:
(1) B+0.5N
(2) Mo+0.5W
(3) C+Mn/6+Si/24+Ni/40+Cr/5+Mo/15+V/14
In this case, it is possible to set the V content at 0.23 to 0.27 mass %, to set the Nb content at 0.01 to 0.06 mass %, to set the Nb equivalent defined by formula (4) given below at 0.15% or less, and to perform the tempering step at 720 to 780xc2x0 C.:
(4) Nb+0.4C
It is also possible to set the V content at 0.23 to 0.27 mass %, to set the Ti content at 0.005 to 0.01 mass %, and to perform the tempering step at 720 to 780xc2x0 C.
Further, it is possible to set the v content at 0.25 to 0.3 mass %.
In the manufacturing method of the present invention, a melt containing components of the specified composition is cast in a sand mold, followed by annealing the resultant ingot. Then, the ingot is subjected to a normalizing treatment (solution treatment).
In the cooling step during the casting, V, Ti and Nb remain as large and coarse carbonitride. In the present invention, these large and coarse carbonitrides are dissolved in the austenite matrix by the normalizing treatment. If the temperature during the normalizing treatment is set lower than 1030xc2x0 C., it is difficult to dissolve the large and coarse carbonitrides in the austenite matrix. On the other hand, if the temperature during the normalizing treatment exceeds 1070xc2x0 C., the matrix falls outside the austenite single phase region, with the result that the metal texture obtained after the hardening step is rendered nonuniform. Under the circumstances, the temperature in the normalizing step is set to fall within the range of between 1030xc2x0 C. and 1070xc2x0 C.
After the normalizing treatment, a tempering treatment is applied to the cast material. In the present invention, where v alone is added without adding Ti or Nb, the tempering temperature is set to fall within a range of between 680C and 7400xc2x0 C. If the tempering temperature is not lower than 680xc2x0 C., the carbonitride of V can be finely precipitated, and it is possible to ensure a sufficient amount of precipitation. If the tempering temperature exceeds 740xc2x0 C., however, the precipitation density of the carbonitride of V tends to be lowered.
On the other hand, where Ti or Nb is added together with V, the tempering temperature is set to fall within a range of between 720xc2x0 C. and 780xc2x0 C. In this case, the carbonitride of Nb or Ti can be finely precipitated, and it is possible to ensure a sufficiently large amount of the precipitation. Where the tempering temperature is lower than 720xc2x0 C., however, it is difficult to ensure a sufficiently large amount of precipitation of fine carbonitride of Nb or Ti. On the other hand, if the tempering temperature exceeds 780xc2x0 C., the A3 transformation temperature is approached or is exceeded. In this case, the texture stability is lowered. Alternatively, the tempering treatment is excessively applied so as to impair the mechanical properties. Under the circumstances, where Ti or Nb is added together with V, the tempering temperature is set to fall within a range of between 720xc2x0 C. and 780xc2x0 C.